The present invention relates generally to a method for manufacturing a semiconductor device, and more particularly, to a method for separating patterns and forming fine patterns of a semiconductor device, in which the distance between individual elements is reduced so as to be below the optical resolution.
The integration density of semiconductor memory devices (particularly, DRAM memories) has been progressively increasing by a factor of four approximately every four years. For example, while the four megabyte (4 Mb) DRAM is being mass-produced, the 16 Mb DRAM is being prepared for mass-production, and 64 Mb and 256 Mb DRAMs are in the stage of research and development. As a result of this quadrennial quadrupling of integration density, the actual area of the memory cells of such memory devices has been reduced by about 30%.
The higher integration density of semiconductor memory devices has been facilitated by two significant factors. More particularly, it has been facilitated by a decrease in the minimum design rule, which entails a reduction in the size of the features etched into the various layers of the semiconductor memory devices, i.e., the number of patterns which can be etched in a predetermined area is increased. In this regard, the reduction of the size of the pattern itself is significant. However, minimizing the distance between the patterns, that is, between individual elements, also contributes much to achievement of high integration density.
In 64 Mb and 256 Mb DRAMs, the separation interval between the individual elements should be reduced to about 0.1-0.5 .mu.m. The separation intervals capable of being achieved by present optical resolutions vary depending upon the type of light used. However, the separation interval is generally about 0.4-0.6 .mu.m. More particularly, the minimum design rule can be determined in accordance with the following equation (1): EQU Lm=.lambda./NA, (1)
wherein Lm denotes the minimum linewidth, .lambda. is the wavelength of the light utilized in the photolithographic processes, and NA is the aperture ratio of the lens employed in the optics system. Most of the photolithographic equipment which is currently used in integration circuit processing utilizes ultraviolet rays (.lambda.=0.2-0.4 .mu.m) as a light source, in which case the minimum linewidth can be reduced by increasing the aperture ratio of the lens. However, the minimum linewidth obtained by current technology is 0.4-0.6 .mu.m.
FIGS. 1 to 3 are sectional views showing a conventional method of separating patterns of a semiconductor device, in which a photolithography process and etching process used for separating patterns (e.g., individual elements) are shown.
With reference to FIG. 1, an insulating interlayer 12 for isolating a semiconductor substrate 10 from a conductive layer, that is, from individual elements, is formed on the entire surface of substrate 10 and then, a first-to-be-patterned-layer 20, for example, a conductive material layer for forming the individual elements (i.e., DRAM word lines, bit lines, storage electrodes, etc.), is formed thereon.
With reference to FIG. 2, after a photoresist film is coated on the first-to-be-patterned-layer 20 by a spin coating method, a pattern-producing mask (not shown) is placed over the photoresist film. Subsequently, the mask is exposed to a light source, whereby a light-transmitting portion and a light-masking portion are produced on the photoresist film in accordance with the pattern formed on the mask. The light passing through the light-transmitting portion is projected on the photoresist film, thereby polymerizing the molecules of photoresist film (for negative photoresist film) and rendering them insoluble in a developer, or decomposing the molecules of the photoresist film (for positive photoresist film) and rendering them soluble in a developer. Subsequently, after removing the mask, the photoresist film is soaked in a developer, and the soluble portion of the photoresist film is removed and the insoluble portion of the photoresist film remains, to form a photoresist film pattern 30 as shown in FIG. 2. (This is a photolithography process.)
With reference to FIG. 3, a first pattern 22 is formed by anisotropically etching (for example, RIE etching) the first-to-be-patterned layer 20 using the photoresist film pattern 30 as an etching mask. (This is an etching process.)
In the above-described conventional method for separating individual elements of a semiconductor device, since distance L' between the patterns is determined in accordance with the resolution of the light (typically, ultraviolet rays) used as a light source, reduction of the distance L' is possible only by enhancing the optical resolution. Accordingly, the conventional method is not appropriate for the development of 64 Mb and 256 Mb DRAMs in which the distance between the patterns should be reduced to about 0.1 .mu.m to 0.5 .mu.m.